Patent Literature 1 discloses a transistor drive method. In this drive method, a voltage of a power supply called a gate-off power supply connected to a main power supply is applied to a gate when a control power supply for applying a voltage to the gate is shut down. The occurrence of transistor arm shorting or the like is thereby avoided when the control power supply is shut down. Transistors used therein are depletion-type (normally-on) transistors.